Over the eight years the study covered, the modeling that the team conducted suggested that the accuracy of a group's flight path should increase by about 5.5 percent for every additional year in the oldest bird's age. For groups that included a 7-year-old bird, that would imply a 38 percent improvement over a group composed entirely of first-timers.
The team's field data suggest that at least for a crane's first five years, it indeed gets better each year at staying on course.
"There's continuous learning over many years of migration" as information is passed from the older birds to the younger birds, Mueller says.
And unlike many other bird species, the instructors aren't parents, they are complete strangers, suggesting a strong cultural component to the activity of learning, says Sarah Converse, an ecologist at the US Geological Survey's Patuxent Wildlife Research Center in Laurel, Md.
Conservation of migrating whooping cranes remains an ambitious work in progress. The birds are native to North America, but by the 1940s, the number of known individuals had plummeted to 14, Dr. Converse explains, in addition to a couple of populations of nonmigrating whooping cranes.
Breeding programs have rebuilt a population that migrates between northern Canada and the Texas Gulf Coast to more than 250 birds. An effort to reestablish an eastern population run by the Whooping Crane Eastern Partnership has been under way for 12 years. The population started at zero and now hosts more than 100 birds.
But these birds all have been born in captivity at the Patuxent Wildlife Research Refuge and a handful of other breeding centers. At about 6 months old, they are shipped to the Necedah National Wildlife Refuge, about 80 miles northwest of Madison, Wis., to join with other whooping cranes summering there.
There, the youngsters are trained to view an ultralight airplane as one of them, following it as it taxis around the summering grounds. When it comes time to head south, the pilot leads a migration group full of rookies. More recently, the effort has used older birds to guide groups as well.
A one-way trip to the wintering grounds at the Chassahowitzka National Wildlife Refuge, about 20 miles south of Chrystal River, Fla., is 1,140 miles.
Mueller's team used exhaustive amounts of data the Whooping Crane Eastern Partnership has gathered on each bird it has raised – including travel routes and genetic information – for its analysis of annual migrations from 2002 to 2009.